An RF ID tag with a battery power supply or of an active type, which is attached to a merchandise article or the like, or carried by a person, transmits an RF signal at a predetermined frequency that carries an ID and other information related to the article or the person, so that the RF signal is received and the information is read out by a reader device. The read-out information is further processed by a computer or the like, so that the distribution of the article or the action of the person is monitored and managed. The active-type RF ID tag with battery power supply has a larger communication range than a passive-type RF ID tag that receives power from a reader/writer device in a contactless manner, and hence is practical in use. However, the active-type RF ID tag transmits an RF signal in a fixed cycle, has a risk of being tracked by a third party, and hence has a problem in the security. To address this security problem, there has been developed an improved active-type RF ID tag that responds only to a tag ID request transmitted by the reader/writer device.
PCT International Publication WO 97/43740 published on Nov. 20, 1997 describes radio frequencies identification device which includes an integrated circuit including a receiver, a transmitter and a microprocessor. The receiver and transmitter together form an active transponder. The integrated circuit is preferably a monolithic single die integrated circuit including the receiver, the transmitter and the microprocessor. Because the device includes an active transponder, instead of a transponder which relies on magnetic coupling for power, the device has a much larger range.
Japanese Patent Application Publication JP 2000-113130-A published on Apr. 21, 2000 describes an IC tag detection system with low power consumption. This system includes a plurality of IC tags provided with different set times of day. Each IC tag includes a communication circuit, a control unit, a power source unit for supplying power from a battery to them, and time measuring means. Each IC tag performs transmission at each prescribed set time of day. This system also includes a detector for detecting the presence or absence of the IC tags based on the communication with them. The detector has a communication circuit, and determines the presence or absence of reception from them successively at the respective set times of day of the respective IC tags. Since the IC tag receives no inquiry from the detector, the IC tag can avoid useless reaction and battery consumption.
Japanese Patent Application Publication JP 2001-251210-A published on Sep. 14, 2001 describes a method of locking a frequency in a transmitter at each of two nodes in a full duplex link, without using a separate reference oscillator in each node. The method provides locking of transmission frequencies of both nodes in a full duplex link at the same time by utilizing information of a received frequency to tune carrier frequencies of the transmitters. The offset of the carrier frequency of the fist transmitter is detected as the offset of a second corresponding receiver. The second receiver shifts the carrier frequency of the second transmitter, in response to the detected offset, to inform the first transmitter about the detected offset. The first receiver uses the detected offset to correct the carrier frequency of the first transmitter.
International Publication WO 98/16849 which was published on Apr. 23, 1998 describes a system for tracking mobile tags. In the system, cell controllers with multiple antenna modules generate a carrier signal which is received by the tags. The tags shift the frequency of the carrier signal, modulate an identification code onto it, and transmit the resultant tag signal at randomized intervals. The antennas receive and process the response, and determine the presence of the tags by proximity and triangulation. The distance of a tag from an antenna is calculated by measuring the round trip signal time. The cell controllers send data from the antenna to a host computer. The host computer collects the data and resolves them into positional estimates. The data are archived in a data warehouse, such as an SQL Server.
Japanese Patent Application Publication JP 2000-20651-A published on Jan. 21, 2000 describes a reader/writer. The reader/writer is equipped with a synthesizer having a carrier wave generating means that can change the frequency of a generated carrier, has a modulator for modulating the carrier generated by the synthesizer, and communicates an RF signal carrying information with an ID tag by transmitting the RF modulated carrier via a transmission amplifier or a circulator from an antenna. The synthesizer includes, for example, an oscillator for generating a carrier at a variable frequency, an adjustment means for changing an oscillation frequency of the oscillator in response to the input from the outside by controlling the oscillator via a loop filter, and a crystal oscillator for supplying a signal at a predetermined frequency to the adjustment means. This prevents carrier interference between the reader/writers.
Japanese Patent Application Publication JP 2000-187711-A published on Jul. 4, 2000 describes a reader/writer device. The reader/writer device transmits an anti-collision command, receives data from a tag in each time slot, and allocates simple ID numbers ID1-ID4 to respective unique ID codes AAAA, BBBB, CCCC and DDDD of the tags. Only these simple ID codes ID1-ID4 are transmitted to the host device as normal responses. The host device obtains the simple ID numbers, and issues read or write communication commands. This reduces the time and amount of communication with the host device.